Bottom-Current and Wind-Pattern Changes As Indicated by Late Glacial and Holocene Sediments from Western Lake Geneva (Switzerland)

0012-9402/03/01S039-10 Eclogae geol. Helv. 96 (2003) Supplement 1, S39–S48 Birkhäuser Verlag, Basel, 2003

Bottom-current and wind-pattern changes as indicated by Late Glacial and Holocene sediments from western Lake Geneva (Switzerland)

STÉPHANIE GIRARDCLOS1, 2, IRA BASTER1,3, WALTER WILDI 1, ANDRÉ PUGIN1, 4 & ANNE-MARIE RACHOUD-SCHNEIDER 5

Key words: limnogeology, seismic stratigraphy, Lake Geneva, Late-Glacial, Holocene, isopachs, bottom currents Mots-clés: limnogéologie, stratigraphie sismique, Lac Léman, Tardiglaciaire, Holocène, isopaques, courants profonds.

ABSTRACT RESUME

L’histoire sédimentaire tardiglaciaire et holocène de la zone des Hauts-Monts The Late-Glacial and Holocene sedimentary history of the Hauts-Monts area (partie occidentale du Lac Léman, Suisse) est reconstruite grâce à la combi- (western Lake Geneva, Switzerland) is reconstructed combining high resolu- naison d’une stratigraphie sismique à haute résolution et de datations de tion seismic stratigraphy and well-dated sedimentary cores. Six reflections and carottes de sédiment. Six réflecteurs et unités sismiques sont définis et seismic units are defined and represented by individual isopach maps, which représentés sous forme de cartes isopaques individuelles, qui réunies, établis- are further combined to obtain a three-dimensional age-depth model. Slumps, sent un modèle âge-profondeur tridimensionnel. Des slumps, des zones ‘sour- blank areas and various geometries are identified using these seismic data. des’ et la géométrie des réflecteurs sont identifiés à l’aide des données sis- The sediment depositional areas have substantially changed miques. throughout the lake during the end of the Late-Glacial and the Holocene. Les principales zones de dépôt sédimentaire ont considérablement These changes are interpreted as the result of variations in the intensity of changé durant la fin du Tardiglaciaire et l’Holocène. Ces changements, inter- deep lake currents and the frequency of strong winds determining the distribu- prétés en terme d’intensité des courants lacustres profonds et de fréquence des tion of sediment input from the Versoix River and from reworking of previ- forts vents, déterminent la distribution des apports sédimentaires de la Versoix ously deposited sediments within the lacustrine basin. et du remaniement des sédiments précédemment déposés au sein du bassin la- The identified changes in sediment distribution allowed us to re- custre. construct the lake’s deep-current history and the evolution of dominant strong Ces modifications dans la distribution des sédiments ont permis de wind regimes from the Preboreal to present times. reconstruire l’histoire des courants lacustres profonds et l’évolution des régimes dominants des fort vents du Préboréal à nos jours.

1.- Introduction

Lake sediments are considered to be among the most sensitive mate changes allowing inter-site comparisons over distances of archives of environmental and climate changes on the conti- hundreds of kilometres (Eicher & Siegenthaler 1976, Kelts nents. The size and morphology of lakes and their associated 1978, Lotter et al. 1992, Niessen et al. 1992), and eventually to features greatly determine the sediment record (Håkanson the marine and ice core records. Depending on the part of the 1977, Pourriot & Meybeck 1995). In particular, large lakes lacustrine basin –lake-bottom or delta/coastal areas- the sedi- register major events averaging long-term environmental mentary record is dominated by internal or external processes, changes. Their sediment infills, therefore, record regional cli- respectively.

1 Institut F.A. Forel, route de Suisse 10, CH-1290 Versoix (Switzerland) 2 Present address: Geologisches Institut – ETH Zentrum, CH-8092 Zürich (Switzerland) 3 Present address: Departement of surface water - EAWAG, CH-8600 Dübendorf (Switzerland) 4 Illinois State Geological Survey, 615 E. Peabody, Champaign, IL 61820 (USA) 5 Tattes d’oies 19, CH-1260 Nyon (Switzerland) Corresponding author: [email protected]

Bottom currents and wind patterns Lake Geneva S39 Fig. 1. Location map of Lake Geneva and 3D representation of the Hauts-Monts study area. Bathymetry is indicated in meters.

The sedimentary infill of numerous perialpine lakes has been 20°C. The lake basin is traditionally and geographically divid- studied during the last several years in Switzerland (Gaillard & ed into two components: the large, 300-m-deep “Grand-Lac” Moulin 1989, Lister 1988, Lotter 1999, Niessen & Kelts 1989, and the elongated, 50-70-m-deep “Petit-Lac”. Schwalb 1992, Sturm & Matter 1978, Wohlfarth & Schneider The studied “Hauts-Monts” zone is located in the 1991), France (Chapron 1999, Magny 1992, Van Rensbergen well-oxygenated and mixed waters of the Petit-Lac. The sedi- et al. 1998) and Austria (Schmidt et al. 1998, Wessels 1998). mentation in this area is influenced by the Versoix River However, the use of both high resolution seismic and sedimen- mouth, which has an estimated median flow rate of 3 m3/s tary studies is still scarce. Lake Geneva, the largest freshwater (Département de l’Intérieur 1996), and by a large underwater basin in western Europe, is part of the Rhône River system and promontory 9-14 m below the present lake level (alt. 372.05 cuts across the Alpine foreland basin from the Alps to the Jura m, Fig. 1). This subaquatic platform is a topographycally high Mountains (Fig. 1, Wildi & Pugin 1998). Its bedrock morphol- piece of Molasse bedrock that resisted glacial erosion during ogy and Pleistocene glacial sediment infill have been previ- the last ice ages. ously described by Vernet & Horn (1971), Vernet et al. (1972), This paper presents the Late-Glacial and Holocene Wildi et al. (1997) and Moscariello et al. (1998). Detailed paleoenvironmental evolution of a bottom-current dominated studies of the Holocene sediment record have also been pub- area in western Lake Geneva. Based on both seismic and sedi- lished by Loizeau (1998), Moscariello (1996), Girardclos mentological approaches we have reconstructed the changes in (2001), Baster (2002) and Baster et al. (this volume). the geometry, lateral extent and thickness distribution of indi- Lake Geneva is monomictic and mesotrophic, and its vidual seismo-stratigraphic units throughout time and the asso- surface-water temperature varies yearly between 5°C and ciated paleoenvironmental history of the lacustrine basin.

S40 S. Girardclos et al. 2.- Methodology 20299). The other reflections’ ages (reflections n° 9/10 and 14) were estimated compiling and correlating previous palynolog- 2.1. Seismic data acquisition and processing ical analyses (Burrus 1980, Châteauneuf & Fauconnier 1977, Lüdi 1939, Moscariello 1996, Reynaud 1981). The time scale In 1997, 200 km of seismic profiles within a 4 x 3.5 km area displayed in Fig. 2 is based on chrono- and biozones defined have been acquired using a high resolution BATHY-1000 for the Swiss Plateau (Ammann et al. 1996, Lotter 1999). echosounder (Girardclos 2001), connected to a Differential Unit c1* represents the end of the Late-Glacial, from Global Positioning System (DGPS) with an accuracy of ±2-10 the end of the Bølling to the beginning of the Preboreal (Fig. m. The dominant wave frequency of this seismic system is 1.5- 2). Unit c2 spans the first half of the Holocene, from the Pre- 2 kHz. Following the Rayleigh’s criterion, the calculated verti- boreal to the end of the Younger Atlantic. Unit c3 encompass- cal resolution of the echosounder data is 0.25 m (Girardclos es the entire Subboreal, the Older Subatlantic and the very be- 2001). After standard seismic processing, the lines were digi- ginning of the Younger Subatlantic. All the remaining units tally analysed with the PC software SEISVISION 4.0 of GEO- d1, d2 and d3 represent the Younger Subatlantic. The high res- GRAPHIX LTD. The seismo-statigraphic analyses was exported olution of the Younger Subatlantic palynological data (Girard- into ASCII files as x,y,z coordinates and converted from time clos 2001) enabled us to estimate that unit d1 corresponds to values into depths. Each data set, representing a major seismic the late Middle Ages, unit d2 to Modern Times (16th - 18th reflection, was then interpolated into digital grids (i.e. sur- century) and the uppermost unit d3 to the contemporary epoch faces) with a radial basis function. These grids were plotted th with either a 0.25- or a 0.5-m contour interval and then com- (19 century to present). bined to calculate the thickness of each sedimentary sequence. Baster et al. (this volume) provide additional details on the seismic data acquisition and processing. 3.- Seismic stratigraphy

2.2. Dating of the seismic units 3.1. Reference echosounder profile

The correlation of the seismic reflections with 12 sedimentary A close-up of the eb1 echosounder profile (Fig. 3) summarises cores, retrieved between 1997 and 1999, allowed the age de- the seismo-stratigraphic units identified in the Hauts-Monts termination of reflections n° 5, 6, 8 and 16 combining various area. This seismic line is oriented NW-SE and located in the methods: palynological analyses (Rachoud-Schneider 1999), 60-65-m-deep centre of the Petit-Lac basin, about 1 km from 137Cs activity measurements and one AMS-14C date (ETH n° the Versoix River mouth. The semi-continuous to continuous reflections have medium to high amplitudes with moderate fre- quencies. Unit (c1) is bounded by the reflections n° 16 (bottom) and n° 14 (top) and is characterised by a parallel layering. Reflection n° 14 is truncated in the SE part of the profile. The upper sequences (seismic units c2 to d3) are discordant with unit c1 and form lakeward dipping foresets that downlap and onlap onto unit c1. The generally semi-continuous to continuous reflections and the well-defined stratification of the seismic sequences indicate a lacustrine origin for these units. The trunca- tion of reflection n° 14 in the SE part of the profile reveals a past erosional event. The observed downlap and onlap prograding sequences from NW to SE (above unit c1) are usually interpreted as prograding delta or fan geometry (Badley 1985). Various directions of the progradation can be observed and indicate that another process is shaping this geometry (see Sec- tion 4).

3.2. Thickness maps

Fig. 2. Late-Glacial and Holocene ages of the Hauts-Monts area seismic re- The thickness of the different seismic units c1 to d3 were esti- flections and units inferred from palynological analyses of 12 sediment cores mated from the 200 km of echosounder profiles and isopachs and estimated from previous palynological data in western Lake Geneva are shown for each sediment sequence separately (Fig. 4). Ad- (Burrus 1980, Châteauneuf & Fauconnier 1977, Lüdi 1939, Moscariello 1996, Reynaud 1981). ditional features (slumps, seismic blank areas, reflections

Bottom currents and wind patterns Lake Geneva S41 Fig. 3. Close-up of the interpreted echosounder profile eb1. Seismic units c1 to d3 are separated by reflections (n° 5-16) that are traced with different line patterns. On this profile, units c2 to d3 are prograding SE with downlap and onlap geometry. The truncated seismic reflection n°14 indicates a past erosional event. geometry), analysed from 2D echosounder data and the in- Unit c2, Preboreal → end of Younger Atlantic (Fig. 4 b): ferred main sediment supply are also shown on the maps. Unit c2 thickness varies from 0 to 4 m with maximum values Gas blanking located in the deepest part of the basin (NE) and close to the Versoix River mouth (W and SW). The thinnest sediments are A wide blank area in the eastern part of the basin can be distin- found near the 675’000 m2 of non-deposition area. As unit c1, guished in all the thickness maps (Fig. 4 a-f). This area follows this unit is bounded to the north by downlap and onlap geome- the lake slope and extends lakewards away the Versoix River try of its upper reflection (n° 9/10), which becomes truncated mouth. towards the south. The non-deposition area of unit c2 deter- mines irregular lobes and its centre is shifted ca. 300 m SSE Unit c1, end of Bølling → beginning of Preboreal (Fig. 4 a): from the centre of the unit c1 non-deposition area.

This map shows the sediment distribution pattern at the end of Unit c3, Subboreal → beginning of Younger Subatlantic (Fig. unit c1. The layer thickness varies from 0 to 2.5 m. 2D seismic 4 c): profiles indicate that sediment slumping started from two places in the NW part of the basin during this interval. The Unit c3 thickness ranges from 0 to 4.5 m, thickenning in the volume of the northern slump represents more than 4 x 106 m3 NE part of the basin, on the northern slope of the Hauts-Monts of sediment. The thickest stratified sediments are localised promontory and SE of the Versoix River mouth. In contrast to along the NW-SE area facing of the northern slump. An exten- units c1 and c2, unit c3 is bounded by downlap and onlap sive non-deposition area of 838’000 m2 appears at the top of geometries along the entire perimeter of the non-deposition unit c1. Along its northern side, the top of unit c1 is bounded area. The non-depositional area has the same centre as unit c2 by the downlap geometry of reflection n° 14, which becomes but is about half the size (394’000 m2). truncated along its southern side. Unit d1, late Middle Ages (Fig. 4 d):

The unit d1 isopachs indicate substantial sedimentary changes

S42 S. Girardclos et al. Fig. 4. Thickness maps of the Hauts-Monts area sequences showing changes in the sedimentary depositional patterns. The underlying thin lines are the approxi- mate present bathymetry contours of the lake basin. a) Unit c1 end of Bølling ® beginning of Preboreal, b) Unit c2, Preboreal ® end of Younger Atlantic, c) Unit c3, Subboreal ® beginning of Younger Subatlantic, d) Unit d1, late Middle Ages, e) Unit d2, Modern Times (16th-18th century), f) Unit d3, contemporary epoch (19th century - present).

at the beginning of the Subatlantic biozone. The sediments are the promontory at 35 to 45 m depth below present lake level. comparatively thicker in the northern part of the basin forming a sediment tongue NE of the Versoix River mouth. The non- Unit d2, Modern Times (16th-18th century) (Fig. 4 e): depositional area is further reduced in comparison to the previous units comprising an area of only 195’000 m2. It is bounded The most recent situation (unit d2) is similar to that of the late by the downlap and onlap geometry of unit d1. A long trunca- Middle Ages (unit d1), but the difference in thickness between tion of the top reflection (n°6) surrounds the northern side of the northern and the southern part of the basin is less pro- nounced. The non-depositional area of unit d2 is again smaller (147’000 m2) than that of unit d1 and it is delimitated with the

Bottom currents and wind patterns Lake Geneva S43 downlap and onlap geometries of unit d2. An extensive trunca- the lake basin starts at the river plume, either through over- tion of the top reflection (n°5) encircles the NE side of the flow, interflow or underflow, depending on the density differ- promontory at depths of 40-55 m like unit d1. ence between river and lake water (Giovanoli 1990, Sturm & Matter 1978). For overflow and interflow, further sediment Unit d3, contemporary epoch (19th century - present) (Fig. 4 transport occurs by internal lake currents. Reworked sediments f): from lake platforms are also transported toward the deepest part of the lake basin and deposited near the Hauts-Monts plat- Unit d3 is 0 to 4 m thick and the sediment distribution is com- form by the same way. These processes are attested by terres- pletely different from the previous one. The sediment supply trial and shelly layers in Younger Subatlantic sediments cores of the Versoix River is oriented once again SE, 90° from the (Girardclos 2001). main lake flow. Because of the limited vertical resolution of Today’s measurements in the central part of the our seismic data, the unit d3 non-deposition area could not be Hauts-Monts basin indicate the development of wind-parallel accurately determined. However, previous work (Girardclos lake currents at the surface and counter-currents in deep water. 2001, Fig. 5.22 and 5.23) shows that the thick SW sedimentary During the Versoix flooding events, the main sediment trans- sequences contain two truncated sub-units, suggesting a proba- port is generally oriented opposite to the dominant wind direc- ble non-deposition area as large as that of unit c2. tion and can be explained by strong deep currents in this particular area (Ulmann et al., this volume). Therefore, the origin of such deep water currents are certainly due to strong winds. 4.- The Late Glacial and Holocene sediment distribution: At this location (-65 m depth), other types of water movements Discussion (seiches, horizontal or vertical mixing, etc.) cannot explain such geographical and temporal extent of the erosion, non-de- 4.1. Limnological processes position and transport of sediments. Indeed Lake Geneva surface seiches –as stationary waves- generate only small water Sediment distribution in lacustrine basins is determined by displacements and velocities, whereas stronger internal seiches both input and internal dispersion processes. The main external at the thermocline level have their highest amplitudes and ve- sediment source of the study area is the Versoix River (Ul- locities near the coast (Lemmin 1998). Vertical and horizontal mann 2000, Ulmann et al., this volume). Increasing input can mixing, which are small scale water movements, occur during be further linked to flood events from rain and melting snow. a particular thermal stratification state of the lake but are ulti- The lake’s autochthonous sedimentation, biologically induced mately due to the turbulence effect of the large scale water dis- by coastal organisms, mainly occurs on shallow lake flat areas, placements like currents (Lemmin 1998). where strong winds rework the deposited sediments. These Presently, heavy rain and resulting river floods are platforms, are up to 10 m deep and are particularly well devel- generally linked to barometric depressions and winds blowing oped along the NW lake shore and on the Hauts-Monts from the SW, whereas strong NE winds are associated with promontory due to Molasse bedrock and ice age deposits that maximum fetch and sediment reworking on the platform. It resisted past glacial erosions (Service Cantonal de Géologie has been, therefore, postulated that the recent sedimentation 1974) (Fig. 1). Sediment distribution from the river mouth into SW and N-NE of the Versoix River mouth is linked to SW and NE winds, respectively (Ulmann et al., this volume). The

Table 1. Summary of the primary sedimentological features and evolution through time of the main parameters regulating sediment distribution in the Hauts-Monts area.

S44 S. Girardclos et al. chemical composition of the sediments and the distribution of area indicates a decrease in the bottom-current action. terrestrial plant debris, algae and molluscs support this interpretation (Ulmann et al., this volume). Unit d1, late Middle Ages (Fig. 4 d): Deep lake currents associated with dominant wind regimes, therefore, seem to be the main cause of sediment dis- The dominant sedimentation area follows the lake slope along tribution in the study area, and leads us to the following inter- the platform NE of the Versoix River mouth, indicating either pretations. deviation of the river plume or intense reworking on the lake platform NW of the basin. In any case, the sediment load is 4.2. Interpretation of the sediment distribution oriented in a direction that is opposite to the main lake flow. Only wind-induced currents that have modified in turn the lake Based on these preliminary considerations, the following caus- circulation can explain this sediment distribution. The in- es regulating sediment distribution and its seismic expression creased thickness of unit d1 (0-6 m) during this short time in- are proposed: terval points towards a sharp rise in sedimentation rate, which is most probably related to the heavy deforestation of the Gas blanking: catchment area that increased both runoff and denudation processes during the late Middle Ages (Le Roy Ladurie 1983). The blank area is attributed to the presence of methane (Badley 1985, Fader 1997), most probably produced by the degradation th th of the organic matter from the Versoix River (Ulmann et al. Unit d2, Modern Times (16 -18 century) (Fig. 4 e): 2002, this volume). The presence of gas in the study area is also attested by gas bubbles traces in sediment cores (Girardc- The unit d2 sediment distribution is similar to that of unit d1. los, 2001). Since gas can migrate upwards through the sedi- Sedimentation rates appear to be lower than those of unit d1 (0 ment layers, it is not possible to assign an age to this layer. to 1.5 m thick). This indicates a new decrease in the bottom- current action in the centre of the basin, and a reduction of the Unit c1, end of Bølling → beginning of Preboreal: (Fig. 4 a): Versoix delta sediment supply. The opposite orientation of the Versoix River sediment load to the surface lake flow suggests The high sediment thickness NE of the study area is obviously that wind-induced currents dominated the lake circulation in linked to a major slump on the northern lake slope that may this part of the basin. have triggered a gravity flow that crossed the lake basin and th later settled onto the lake bottom to form a homogenite Unit d3, contemporary epoch (19 century - present) (Fig. 4 (Chapron et al. 1999). Storms or earthquakes can trigger such f): instabilities (Mulder & Cochonat 1996; Postma 1984). The non-deposition area NW of the Hauts-Monts platform is the The sediment supply from the Versoix River is located SW of geographical representation of the stratigraphic geometry al- the river mouth. This observation is consistent with studies of ready discussed for the eb1 echosounder profile (Fig. 2). It is the recent Versoix River sediment distribution and can be ex- most probably the result of strong bottom currents that prevent plained by deep counter currents originating from SW winds sediment deposition. (Ulmann et al. 2002, this volume). In Corsier Bay (Girardclos 2001), the similarity of the truncated geometry between unit Unit c2, Preboreal → end of Younger Atlantic (Fig. 4 b): d3 and units d1 and d2, indicates that heavy erosion probably occurred during the contemporary epoch. Its 35-55 m depth The isopachs indicate increased sediment supply by the Ver- suggests that this erosion resulted from deep erosive currents soix River. The sediments are distributed at the bottom of the along the Corsier Bay slope. slope NE and SE from the river mouth. The decrease in the 4.3. Chronological synthesis of the currents action on the sed- non-deposition area at the foot of the Hauts-Monts platform iments and its 300 m shift to the SSE indicate a decrease in the bottom currents and a probable change in lake circulation patterns. The main interpreted sedimentological features (cf. previous paragraph) are presented chronologically in Table 1 (col. 1 to Unit c3, Subboreal → beginning of Younger Subatlantic (Fig. 3). Using the erosion patterns and the variation in the non-de- 4 c): position area, the intensity of past deep lake currents was reconstructed for the Holocene (Table 1, col. 4). Unit c3 is characterised by the ongoing sediment input from As previously explained, the present distribution of the Versoix River in the SW part of the study area and by the the sediment in the Hauts-Monts area is due to the strong wind sediment input from slumping of the Hauts-Monts slopes in regimes and their resulting meteorological conditions (rain, the NE lake basin. The sharp decrease of the non-deposition etc.). Based on the model for the present situation, it is possi-

Bottom currents and wind patterns Lake Geneva S45 ble to define the relative frequency of strong winds (Table 1, cause the Lake Malawi and Mediterranean contourites have col. 5) and to reconstruct dominant past wind regimes (Table not been dated yet, they cannot be used as possible proxies of 1, col. 6) for the western Swiss Plateau. climate change. It is, therefore, difficult to compare our obser- vations and interpretations with previously published climate or bottom-current related records. Thus, our model showing 5.- Conclusions changes in frequency and magnitude of strong wind regimes represents the first attempt to define wind pattern evolution Our results show that lake currents have influenced the sedi- during the Holocene not only in the western Swiss Plateau but ment distribution of the Hauts-Monts area since the very be- also in other areas of the globe. ginning of the Preboreal (end of unit c1). In addition, the iden- Several questions remain open and need further in- tified current action may have been initiated during the lower- vestigation to constrain our conclusions. One option to test ing of the lake level by at least 20 m between the Oldest Dryas them would be to quantify and model the 3D current velocity and the Preboreal (Gallay & Kaenel 1981), as well as with distribution within the modern lake basin. The latter combined changes in weather pattern most probably associated with the with a precise determination of the different sediment source early Holocene major climate warming. areas will allow a more accurate reconstruction of the mecha- The particular topography of the Hauts-Monts platform which nisms behind past sediment distribution and their triggering narrows the lake basin, certainly accelerates the existing bot- forces. This may provide a unique reconstruction of former tom currents and thus enhances their effect on the sediments. wind directions and the associated atmospheric circulation pat- This could explain the infrequency (or even uniqueness ?) of terns over the Swiss Plateau. the observed sedimentological features in Lake Geneva at depths greater than -60 m. This hypothesis is supported by Acknowledgments similar bottom morphologies (topographic constrictions, nar- row passages and confining slopes) observed in the central We would like to thank I. Hadjas for AMS-14C dating; M. Beres who careful- Mediterranean Sea that control bottom-current activity ly checked the English; and D. Ariztegui and C. Beck for their useful reviews. (Marani et al. 1993). 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